1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride-based polymer, and, in particular, relates to a process for producing a vinyl chloride-based polymer in which, when the vinyl chloride-based polymer is produced in a polymerization vessel equipped with a reflux condenser, foaming of the polymerization reaction liquid, which may occur due to heat removal using the reflux condenser is suppressed, enabling production of the target vinyl chloride-based polymer without any deterioration in quality.
2. Description of the Prior Art
In the production of vinyl chloride-based polymers, reduced polymerization times are being developed to improve the productivity. In one such technique, a method is employed in which the polymerization reaction heat is removed using both a polymerization vessel jacket and a reflux condenser, in order to significantly increase the quantity of polymerization reaction heat removed.
However, in those cases where the production of the vinyl chloride-based polymer is conducted via a suspension polymerization in an aqueous medium, and an aqueous material with surface activity (for example, a partially saponified polyvinyl alcohol or a cellulose ether) is used as a dispersant, if heat removal using the reflux condenser is performed beyond a certain level, then a problem arises in that foaming of the polymerization reaction liquid occurs, making it impossible to obtain a polymer of uniform quality with the targeted levels of particle size distribution, porosity and bulk specific gravity.
Furthermore, if the foaming becomes even more severe, then the polymerization reaction liquid may erupt up inside the reflux condenser, causing particles of polymer to become deposited on the inside of the condenser, and these particles may then remix with the reaction liquid, causing, in addition to the problems described above, further deterioration in the quality of the polymer, including an increase in fish eyes and foreign matter within the molded polymer film.
On the other hand, a process for suppressing the foaming associated with heat removal by a reflux condenser by adding an ethylene oxide/propylene oxide copolymeric polyether (number average molecular weight: 2000 to 9000) as an antifoaming agent is already known. Japanese Post-Examination Patent publication (kokoku) No. Sho 57-17003 (JP57-17003B) discloses a process for suppressing foaming of the polymerization reaction liquid, and suppressing any deterioration in quality due to increases in fish eyes and so on, by adding approximately 0.5 to 200 ppm of the above copolymeric polyether based on the vinyl chloride.
However, based on investigations by the inventors of the present invention, it became evident that this process can result in quality deterioration in terms of factors such as the bulk specific gravity and the volume resistivity of the product polymer.